In the preparation of vinyl polymer and copolymer latexes of vinyl and vinylidene halides polymerized alone or with one or more copolymerizable monomers, a mix is used which, in a preferred embodiment, comprises vinyl chloride monomer, water, emulsifier, a long straight chain alcohol, and an initiator. This mix is initially premixed, then homogenized, and pumped to a polymerization reactor where the vinyl chloride monomer is polymerized by means of emulsion polymerization technique. Crude latex is formed in the polymerization reactor and successively conveyed to a blend tank where other ingredients are added with mixing, then to a concentrator where the crude latex is concentrated to 40% to 60% solids, and finally to a spray drier, if desired, where the latex is dried and formed into a particulate product. From the spray drier, the particulate product is ground and bagged.
The homogenizing procedure includes the step of passing the mix thrugh a two-stage Manton Gaulin (MG) homogenizer which is a positive displacement reciprocating pump operating at a low flow rate but at high pressure. In passing through a homogenizing stage of the MG homogenizer, the mix under pressure forces open a pre-loaded valve where the annular gap between the valve stem and the seat is about 3 to 4 mils. In passing through the gap, the mix undergoes instantaneous pressure drop causing shearing action and cavitation bubbles. In a two-stage MG homogenizer, the mix passes through two such valves with accompanying pressure drops. When leaving the first valve, the mix strikes an annular impact ring thus further shattering the particles by impact and implosion of the bubbles.
The MG homogenizer described above has three suction valves and three discharge valves which act as check valves. Poor homogenization by the MG apparatus is due mainly to polymer deposition on and around the valves which prevents them from fully opening and closing. This condition of the valves leads to a large decrease in flow rate which drastically reduces velocity through the homogenizing valve. This reduced velocity results in poor homogenization which is evidenced by larger droplets and longer than normal homogenization time. Large pressure fluctuations indicate problems of this nature.
Cavitation and turbulence are the two principal mechanisms of homogenization. Velocity determines intensity of cavitation and turbulence in direct relationship, i.e., the higher the velocity the higher cavitation and turbulence and the better homogenization. Reduced velocity, due to plugged valves, will, therefore, reduce quality of homogenization. Although shear plays a minor role in the MG homogenizer, it is, nevertheless, adversely affected by lower velocity. Cumulative effect of all of these factors is erratic homogenization and unpredictable colloidal stability, which makes it impossible to obtain vinyl latex with reproducible properties. Furthermore, the homogenization method, as presently carried out with MG homogenizer, makes it difficult to control plastisol viscosity to obtain reproducible results since more than two passes of the mix therethrough renders viscosity too high because latex particles are outside of the desired size range and the latex is colloidally unstable.
Colloidal stability of vinyl latexes is judged by the quantity of coagulum in the latex. With the customary emulsion polymerization processes, colloidally stable latexes are difficult to obtain since the latexes usually contain polymer particles of varying size too many of which are too fine or too large. Various proposals have heretofore been made to overcome this difficulty but not with the ultimate success. Most of the time, too much coagulation occurs with the resulting latex containing too much coagulum or partially agglomerated particles which precipitate thus reducing the yield. Inordinate quantity of coagulum in latex also clogs strainers downstream of the polymerization reactor, plugs spray-drying nozzles, and causes other difficulties. Clogging of strainers is a particularly vexing problem since the strainers must be opened for cleaning which releases vinyl chloride monomer into the environment thus, too often, creating EPA violations with respect to the maximum allowable quantity of vinyl chloride monomer in the plant environment.
Colloidal stability of vinyl latexes is also affected by the formation of undesirable polymer build-up on the inner surfaces of the polymerization reactor. This deposit or build-up of polymer on the reactor surfaces not only interferes with heat transfer but also decreases yield and adversely affects polymer quality by producing particles, too many of which are too fine or too large, with the resulting adverse effect on viscosity. This polymer build-up must be removed since, otherwise, more build-up occurs rapidly on top of the deposits already present resulting in a hard, insoluble crust.
In the past, it has been the practice to have an operator enter the reactor and scrape the polymer build-up off the walls, baffles and agitators. This operation was not only costly both in labor and in down-time of the reactor but presented potential health hazards as well. Various methods have heretofore been proposed to remove the polymer build-up, such as solvent cleaning, various hydraulic and mechanical reactor cleaners, and the like, but none has proved to be the ultimate in removal of polymer build-up.
In addition to the colloidal stability difficulties, prior art processes for making vinyl latexes by emulsion polymerization suffer from other disadvantages which include poor reproducibility of good quality latexes and lack of any control of viscosity. It is possible to produce good quality latexes with prior art processes, however, unanticipated periods of disastrous results often are encountered and large quantities of latex is produced which does not meet product specifications. Furthermore, due to strictures of the prior art practices, viscosity of the resulting latex can also suffer unpredictably.